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Preliminary scientific assessment

Expedition 333 achieved its primary objectives regarding the sedimentary sequence. Basement coring was limited to the upper 100 m as a consequence of adverse weather conditions and a technical failure (destruction of polycrystalline diamond compact drill bit [PDC]). Yet, additional cores from across the sediment/basalt interface and from basalt obtained at deeper levels than during Expedition 322 will yield important shore-based results. Temperature measurements with the APCT-3 at all three sites and thermal conductivity measurements on cores yielded precise determinations of heat flow, which are slightly lower than shallow heat flow probe data acquired on the Kumano transect (Kinoshita et al., 2008). Cores filling the gaps between the seafloor and the top of RCB coring during Expedition 322 were recovered, and some overlap was achieved to provide a continuous data set. The sediment/basement interface was recovered again, with ESCS in Hole C0012E and RCB in Holes C0012F and C0012G, and interstitial fluid samples were extracted again from the sediment immediately above the interface. The 8.20 m core taken across the interface with ESCS was of very good quality. Postcruise work on these new samples will complement the analyses performed on Expedition 322 cores, notably with fluid isotopic chemistry and hydrological and mechanical tests.

At the NanTroSLIDE site, coring with HPCS, EPCS, and finally ESCS provided nearly continuous recovery across a thick MTD (our primary target) near its edge. The depth, thickness, age, composition, and structural character of this MTD were determined, as well as that of several thinner intervals of sediment disturbed by slope instability processes. The target depth (350 mbsf) was not drilled, but sufficient recovery was achieved below the thick MTD to 314 mbsf to characterize the underlying sediments as a turbidite sequence holding little evidence for postdepositional remobilization. The primary goals of drilling at Site C0018 were thus reached. Tephrochronology, paleomagnetism, and micropaleontology results are consistent, and good quality dating will be obtained over most of the cored interval in spite of the reworking of sediment inherent to a MTD sequence. The age of the thick MTD can already be bracketed between 0.85 Ma and 1.05 Ma from two ash layers of known age. Most HPCS cores taken within the MTD are of good quality and were sampled for shore-based geotechnical measurements. A thick ash layer lying immediately below the MTD was given special attention.

Data acquired during Expedition 333 address important questions regarding the state of material input to the subduction zone, which were left partially unanswered after Expedition 322. Some results on heat flow, stratigraphy, physical properties, and diagenesis are already well established by the end of the expedition. These, combined with knowledge acquired at other Nankai area drill sites (IODP and ODP) provide a basis for preliminary interpretations and orientation for postcruise studies. Other questions, notably regarding basement and sediment-basement fluid interaction, can only be achieved by postcruise work. Another important aspect of the NanTroSLIDE site is ground-truthing of MTD occurrences, based on seismic reflection images. Coring brought simple answers to several of the questions asked and some unexpected findings. The sedimentological and structural observations lead to refined hypotheses on MTD processes and their interaction with tectonic activity and sedimentation on Nankai margin.

Can a change of physical properties between 200 and 250 mbsf at Site C0011 be related to lithologic variation or diagenesis? Does the same transition occur at Site C0012?

A major change of physical properties is found at ~250 mbsf at Site C0011 and has tentatively been identified between 70 and 90 mbsf at Site C0012. This transition appears as a lithologically determined feature enhanced by diagenesis. The content of discrete ash layers in the sediment decreases below this boundary and an increase of the state of alteration of volcanic glass shards is observed in the remaining ash layers. Siliceous fossils are also present above, and there is a sharp decrease in dissolved silica concentration below the boundary. From an analogy with results obtained after Leg 190, it can be proposed that the process responsible for an anomalously high porosity of the sediment above this boundary is cementation by opal-CT and opal dissolution with precipitation of quartz below the boundary (Spinelli et al., 2007). However, the temperature of the reaction would be unusually low at Site C0011 (currently 25°C, as opposed to 55°C at Site 1173) and, assuming constant heat flow, cannot be explained with the reaction kinetics used by Spinelli et al. (2007); and fresh glass is found again in volcaniclastic sands at a deeper level, associated with high silica concentration in the interstitial water. Furthermore, in the case of Sites C0011 and C0012, the primary silica source is dispersed volcanic glass rather than biogenic opal-A (White et al., 2010). It thus remains questionable that the transition observed is controlled by the kinetics of the opal-A/opal-CT and opal-CT/quartz reactions. An alternate possibility is that opal-CT do not form in sufficient amounts to prevent compaction if the initial concentration of amorphous silica in the sediment is too low. Answering these questions will require postcruise work.

Chronostratigraphy based on paleomagnetism and biostratigraphy suggests the age of the transition from opal-cemented hemipelagic mud with numerous ash layers to compacted hemipelagic mudstone with sparce ash layers at Site C0011 (5.25 Ma) is slightly younger at Site 1177 (4.5 Ma), the reference site for the Ashizuri transect. It is even younger at Site 1173 (3.0 Ma), the reference site for the Muroto transect, where higher heat flow and temperature can account for a more advanced silica diagenesis and result in an upward migration of the transition in the sedimentary column (Spinelli et al., 2007). On the Kumano transect, this transition could represent an initial state of diagenesis.

Is fluid circulation in basement and permeable sedimentary layers influencing heat flow and diagenesis at Sites C0011 and C0012?

Heat flow measured during Expedition 333 is 90 mW/m2 at Site C0011 and 140 mW/m2 at Site C0012, respectively ~20% lower and 30% higher than the heat flow expected from conductive cooling of a 20 Ma lithosphere (Kinoshita et al., 2008). The temperatures extrapolated to basement are, respectively, 79° and 65°C at Sites C0011 and C0012. Large variations of heat flow correlated with basement topography are often reported in zones where off-ridge hydrothermal convection is occurring and may be accounted for by supercritical Rayleigh convection (e.g., Fisher et al., 2003). Such a mechanism could account for the heat flow contrast between Site C0011 located on the flank of the Kashinosaki Knoll where sediment thickness is ~1 km and Site C0012, located near its summit where sediment thickness is 520 m. Whether larger scale fluid circulation in the basement (Spinelli and Wang, 2008), rather than mantle and magmatic processes, should be invoked to explain regionally elevated heat flow in the eastern part of the Shikoku Basin remains an open question.

Data acquired during Expedition 333 provide few new arguments in favor of fluid flow along permeable sedimentary horizons. At Site C0011, a higher Ba concentration is observed at the level of the presumably permeable volcanic turbidite facies (Unit II), as well as slightly higher (Ca and K) and lower (Sr and Li) concentrations of several other elements. These are possibly influenced by lateral flow. However, rigorous assessment of lateral flow will require postcruise modeling with consideration of local equilibrium and diagenetic reactions. This is now possible due to successful heat flow measurements.

How does contrasting interstitial water chemistry at Sites C0011 and C0012 relate with in situ diagenesis, fluid flow, and heat flow?

Temperature and heat flow determinations indicate that the temperature conditions in the uppermost basement at Site C0011 are within the temperature window for the onset of the smectite-illite reaction (~55°–90°C). Unfortunately, coring during Expedition 322 was aborted almost 200 m above the top of basement at Site C0011. Although it is unlikely that the reaction has progressed to a significant extent in any of the cored sections, illitization should be expected in the lower part of Site C0011 and it should continue into the trench where the Shikoku Basin sediments are buried to greater depths. Overall, the temperature conditions at correlative stratigraphic levels do not vary by more than ~15°C between the two drill sites. Consequently, differences in fluid composition observed between the two sites (e.g., increasing versus decreasing chlorinity toward basement) cannot result solely from in situ reactions in the cored intervals but reflect different processes occurring in the lower part of the sedimentary column and, perhaps, basement.

Is magmatic activity heterogeneous in composition and age on a backarc basin basement high? Is alteration of the upper oceanic basement heterogeneous, and how does it influence geochemical and fluid budgets?

These questions can only be answered postcruise, but preliminary analyses do not indicate heterogeneity in composition in the 100 m cored. Early oxidizing alteration localized around fractures and pillow rims may be distinguished from pervasive replacement under reducing conditions of glass and mineral phases (olivine and, often, plagioclases) by saponite and zeolites, which may preclude precise radiometric dating. Core recovery was poor in the upper part of the basement but improved downward as advance was reduced. Although drilling operations during Expedition 333 demonstrate the possibility for the D/V Chikyu to maintain a steady rate of penetration while coring in basement, suggestions for improving efficiency and core quality may be driven by our experience as well as that of Expedition 322.

Other questions at input sites

We already pointed out an important outcome of Expedition 333, which is the description of a transition in physical properties and lithology that has strong similarities (sharp porosity decrease with depth, variation in ash occurrence and alteration state, and variation in silica concentration in the interstitial water) with transitions observed on the Muroto and Ashizuri transects at the facies boundary between the upper and lower Shikoku Basin. The depth of this basin-wide transition is at least in part determined by silica diagenesis (Spinelli et al., 2007) and, as may be expected for a diagenetic front, is diachronous, ranging from 5.25 Ma at Site C0011 to 2.5–3 Ma on the Muroto transect (Moore et al., 2001). On the other hand, Expedition 322 identified important local variations in facies in the middle and lower part of the sedimentary column (>7.5 Ma), which led to the definition of the Shikoku Basin volcanic turbidite facies (Underwood et al., 2010). In this context, it is remarkable that the active décollement of the accretionary wedge is located in the 5–7 Ma interval on the Muroto and Ashizuri transects (Moore et al., 2001) and, arguably, also in the same 5–7 Ma interval on the Kumano transect, based on chronostratigraphic data from the base of the hanging wall at frontal thrust Sites C0006 and C0007 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009) and on the correlation between Site C0011 stratigraphy and seismic profiles (Moore et al., 2009; Underwood et al., 2010). Compared with deposits above and below, this interval may be characterized by fewer (or absent) ash layers and an absence of turbidites. Understanding how the main décollement apparently localizes in a specific sedimentary facies in spite of lateral stratigraphic heterogeneity (or, more specifically, understanding the relationships between the décollement, the diagenetic front above, and the absence or presence of one or several turbidite sequences below) remains an important problem.

Site C0012 appears strongly influenced by mass wasting processes. First, a sedimentation hiatus is found between ~1.2 and ~3 Ma. Second, the sediments below the hiatus down to 90 mbsf are affected by rotation of bedding, with dips up to 60°. Disturbed sediments evocative of MTDs were also found in the lowermost part of the interval cored during Expedition 333, as already noticed at these depths during Expedition 322. A deeper occurrence of an unconformity above slumped sediment at ~9.5 Ma was documented during Expedition 322 (Underwood et al., 2010), and an even longer hiatus is evident along an unconformity within Unit V near the base of the sedimentary section.

Repetition of mass wasting events is very understandable considering the location of Site C0012 immediately above the edge of a major slide scar that is obvious in the bathymetry and seismic data. How this process may affect heat flow data should be taken into consideration in modeling efforts. Mass wasting (possibly combined with fluid movement) may also help explain the high variability of seafloor heat flow probe measurements (Kinoshita et al., 2008). Furthermore, considering Site C0012 as a reference for sedimentation deposited on a structural high in the Shikoku Basin may bring a better understanding of stratigraphy within the accretionary wedge. For instance, Sites C0006 and C0007 at the toe of the accretionary wedge display a hiatus over the same age range (1.2–3 Ma) as Site C0012, below the transition between the Shikoku Basin hemipelagic/pyroclastic facies and the distal trench deposits. We wonder whether the hiatus in the hanging wall of the frontal thrust is related to the original depositional environment, which may also have been a basement high, or is the consequence of later deformation during offscraping.

What is the frequency of submarine landslides near the megasplay fault?

The spacing between intervals where evidence for sediment remobilization was observed in the cores at Site C0018 suggests submarine slope destabilization does not occur systematically during subduction earthquakes. The recurrence of great subduction earthquakes in the Nankai Trough is of the order of 100–200 y, whereas the recurrence of MTDs at Site C0018 is on the 100,000–200,000 y timescale. The rhythm of turbidite deposition on the slope in lithologic Subunit IB below the main MTD is probably on the 1000 y timescale and thus may not be controlled by the earthquake cycle either.

What is the source material of the MTDs? What is the importance of accretionary wedge remobilization versus surficial processes?

The MTDs sampled are remobilized slope sediments in agreement with current seismic interpretation. This, however, does not preclude that the underlying accretionary wedge could have been affected by earlier mass wasting events, or that part of the sediment deposited on this slope originate from erosion of the outer arc high (Strasser et al., 2011). Careful documentation of compositional differences between matrix and clasts should help to answer this question.

What controls type, size, and magnitude of turbidites and MTDs, and how do they change through time?

MTDs display a wide range of thicknesses from about 50 cm to 20–60 m for the thickest deposits, which can be imaged by seismic data. Processes of sediment mobilization may differ depending on the scale of the event and comparison of the thick MTD with smaller scale ones observed in the slope sediment sequence above will bring important insight.

Turbidite type deposits were found at the top of two MTDs and likely result from the deposition of sediments suspended during the event. However, most turbidites found at this site appear unrelated to the MTDs.

How do large MTDs relate with the timing of splay fault activity as inferred from NanTroSEIZE Stage 1 drilling?

All MTDs cored are younger than 1 Ma, and therefore postdate the main phase of activity of this splay fault branch as defined by Strasser et al. (2009) and Kimura et al. (2011).

What are the dynamics of large submarine landslides, and can we infer their tsunamigenic potential?

One important aim of postcruise research will be to understand the relationship between variations in the geotechnical properties of the sediment and occurrence and size of instabilities. We hypothesize that the occurrences of large tsunamigenic submarine landslides, in subduction zones as elsewhere, require specific conditions, such as the presence of discrete layers sensitive to liquefaction at some depth (several tens of meters) below the seafloor. These layers may, for example, correspond to loose granular material or to strain softening clays.

Other questions regarding the NanTroSLIDE site

The finding of such a pronounced transition from turbidite-dominated sedimentation to hemipelagite and ash across the thick MTD at ~1 Ma was not expected. However, this may reflect a change of depositional environment observed in this part of the accretionary wedge slope: there are few sand/silt turbidites over the last 1–1.3 Ma at Sites C0004 and C0008 and over the last 1.6 Ma at Site C0001 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). One possible explanation is that the uplift of the outer arc high at 1.3–1 Ma (Gulick et al., 2010) confined most of the transverse turbidity currents to the Kumano forearc basin and thus shut off sand input to the vicinity of Site C0018. Alternatively, the local evolution observed could correspond to a local change of slope and depositional environment from a perched basin trapping sand transported by turbidity currents to a more steeply inclined slope environment.

Another important outcome of observation on cores is the heterogeneous nature of deformation within MTD deposits. Chaotic, pebbly, or mixed sediment intervals coexist with intervals of coherent bedding dips. Localized deformation structures such as shear zones and fault were also identified. As observations on core are done at a smaller scale than the resolution of seismic profiles, the transparent appearance of the MTD on these profiles can be understood even though they often retain coherent intervals. However, the inner style of deformation of the MTDs cored at Site C0018 is more akin to slumps, dominated by plastic deformation, than to mud flows behaving as fluid.